The present invention relates generally to optical filtering, and more specifically to an active optical lattice filter having an electronically controlled gain for ultra-high bit rate digital signal processing. The invention is also specifically related to optical frequency synthesis, wavelength division multiplexing, wavelength division demultiplexing, and optical switching.
Accurate, efficient, high-speed digital communication systems are increasingly in demand. One advance that has revolutionized the speed and efficiency of digital communication systems is the use of optical signal transmission and processing. Transmitting digital data through the use of light impulses, rather than electrical impulses, is useful for high-speed communications because the available bandwidth of light is many orders of magnitude larger than that of electronics. Additionally, optical signal transmission does not suffer from the same loss of signal over distance experienced with electronic data transmission.
Optical signals are typically transmitted through optical fibers, which are transparent and have a very low loss. Fibers are an example of a dielectric waveguide and may therefore be flexibly routed through buildings and other structures. Many modem communication systems send more than one signal through a single fiber. Multiple signals can travel down a single fiber by use of a process called multiplexing. Wavelength division multiplexing is a procedure by which a transmitter blends signals of different wavelengths into one signal. Increasing the number of signals that can travel through one fiber increases the overall capacity of the communication system. But the systems that receive multiplexed signals must be capable of separating out the individual signals. This is known as demultiplexing.
In addition to a demultiplexing apparatus there is a need for an apparatus capable of compensating for irregularities in the signal including chirp, polarization and frequency dispersion. Frequency dispersion causes higher frequency signals to lag behind lower frequency signals. The wider the frequency bandwidth, the more noticeable the lag. This creates complications at the receiver end of a system because the receiver must be able to sense and remove the undesired lag between frequencies. It is also desirable to perform timing recovery at the receiving end or at specified intervals along the transmission path.
Previous methods of detection and filtration of high-capacity multiplexed optical signals are limited by inherent physical limitations. Many present optical filters rely on the physical movement of lenses and mirrors to filter out individual signals. These mechanical filters are referred to as passive optical filters.
One type of passive optical filter is a deformable mirror device (xe2x80x9cDMDxe2x80x9d). DMDs rely on the physical movement of mirrors to filter optical signals. Accordingly, the speed of a DMD optical signal processor is limited by how quickly the mirrors may be physically manipulated. Additionally, the use of mirrors as a deflecting component, requires a high amount of maintenance and physical tuning in order to maintain the accuracy of the system. Because of these limitations, DMDs are capable of modulating only signal frequencies less than 10 kHz.
Another type of passive optical filter is a Fabry-Perot etalon composed of two or more mirrors facing each other which form an optical cavity or resonator. Due to constructive and destructive interference of the reflected waves, the transmission and reflection of these etalons are frequency, and wavelength dependent. These devices therefore act as filters with magnitude and phase responses.
Another type of resonator structure which can be used as a passive optical filter is a fiber ring resonator. In this device a fiber splitter routes part of the optical signal down a delay path or a series of delay paths. The delayed signals are then remixed with the main signal. The frequency and wavelength dependent constructive or destructive interference of the mixing leads to a filter response which is frequency and wavelength dependent. These devices therefore act as filters with magnitude and phase responses.
Another type of passive optical filter is a grating which is an array or closely spaced line features. Gratings work in either reflection or transmission, and the latter is often preferred in current lightwave systems. An optical signal which encounters a grating is deflected into a direction which depends on its wavelength. In this way demultiplexing may be accomplished by routing different channels into different directions, where they may be detected by different receivers.
An alternative to filtering the optical signal is to detect the optical signal and convert it to an electrical signal. Conventional digital or analog filtering may then be done on the electronic signal. This scheme suffers, however, from much lower bandwidth capability, and will not allow for wavelength division multiplexing, because the spectrum of channels will be simultaneously detected.
Another type of passive optical filter is an acoustical-optic spatial light modulator. This filter uses a RF transducer to launch a compressional wave into a transparent crystal medium, causing a change in the refractive index of the medium. While faster than DMDs, acoustical-optic spatial light modulators are speed-limited and capable of modulating only signal frequencies less than 10 MHZ.
In order to meet the upcoming demand for optical digital signal processors with speed in the GHz range and possibly up to the THz range, a new type of optical filter is needed. Accordingly, it is desirable to have a high-speed optical filter that is not limited by the physical restrictions of previous optical filters.
The present invention provides an active high-speed optical lattice filter with electrically-tunable gain. More specifically, an active optical signal filter comprising two or more gain blocks and a current source independently electrically connected to a first electrode and a second electrode associated with each of the gain blocks, whereby currents can be passed through the gain blocks, the currents being controlled by the current source. Each gain block comprises the first electrode, an n-tppe primary layer electrically connected to the first electrode, an active layer electrically connected to the n-type primary layer, a p-type primary layer electrically connected to the active layer, and the second electrode electrically connected to the p-type primary layer wherein the active layers of the gain blocks are optically connected in series.
Inasmuch as active filters possess variable gains, active filters overcome inherent losses always present in passive systems. The result is a higher quality factor which means sharper frequency definitions, filter skirts, and roll-offs. Further, active control provides for switching and logic capabilities not present in passive filters.
By adding the ability to electronically tune the gain to a conventional optical lattice filter allows programmability and algorithmic self-tuning capabilities, enabling ultra-high bit-rate optical digital signal processing and switching at speeds much faster than can be obtained with current technology.
Because the optical lattice filter has gain and voltage and/or current control, this system is what is known as an active system. An active system can accept an applied control signal, make a quick analysis, then generate an error signal if needed. This gives a system the capability to be adaptively tuned on-line at GHz rates. Furthermore, an active system may have programmability and algorithmic self-tuning to facilitate the high-speed active filtering process.
The present invention further integrates an active optical lattice filter into semiconductor laser material, allowing electronically controlled gain and tunability while preserving bandwidth and quality. Having a lattice filter composed of semiconductor structures, a system of this type may be integrated so far as to accommodate multiple layers. This means a device capable of handling an extremely wide-band signal may be extremely portable. Having a system requiring so little space allows more space for other components, and is often less sensitive to environmental changes.
The present invention centers around an active optical lattice filter. The active optical lattice filter is more generally embodied within an optical signal processor apparatus. The optical lattice filter is capable of filtering an optical signal such as modulated laser beam received through a fiber. The filter of the present invention provides the benefits normally associated with adaptive digital lattice filters, but may be used to operate on signals in the GHz and THz ranges. The active optical lattice filter of the present invention may be operated in a mode which produces oscillations. The parameters of the active optical lattice filter may be adjusted to provide a frequency controlled laser signal. An adjustable precise laser frequency generator is useful for providing a source color in a dense wavelength division multiplexer. The active optical lattice filter of the present invention may also be used in a configuration to construct a dense wavelength demultiplexer. When control signals are applied to rapidly switch the gains from a first set of values to a second set of values, the device""s transfer function switches accordingly, and hence the device may be operated as an optical switch or a voltage controlled optical frequency synthesizer. When configured and operated accordingly, the optical signal processing apparatus of the present invention takes on the form of an optical switching system. All of these embodiments are different configurations of the basic optical signal processor apparatus of the present invention.